GaAs-based semiconductor laser elements are widely used in excitation light sources of optical amplifiers and the like. When the GaAs-based semiconductor laser element is used in excitation light sources, it is necessary that its light output is high. However, when the light output of the semiconductor laser element is increased, following phenomenon disadvantageously occur at the laser end facet of the semiconductor laser element. Firstly there occurs an optical damage, secondly there occurs a corrosion of the laser end facet when the laser element is operated over a long period. It is believed that these phenomenon are caused because of increase of the temperature of the end facet (resonator surface), contraction of the band gap, photo-absorption, recombination current, and a combination of one or more of these.
When the light output of the semiconductor laser element is increased, the optical damage and end facet corrosion become more conspicuous because the light density at the end facet increases. Sometimes the deterioration is so high that the generation of laser is suddenly stopped. In order to overcome these problems, it is desirable to have a semiconductor laser element in which light intensity is reduced only near the end facet.
As a countermeasure, Japanese Patent Application Laid-Open No. 6-188511 discloses a semiconductor laser element which has resonator end facets with different reflectances, and a ridge mesa on an active layer. The ridge mesa is formed in the region except in a region near the resonator end facet on the low reflectance side. At least a part of the region where the ridge mesa is formed is provided with a current non-injection structure.
A cross-section of the semiconductor laser element proposed in the above-mentioned reference is shown in FIG. 9. A cross section of the semiconductor laser element along the line A—A shown in FIG. 9 is shown in FIG. 10. This semiconductor laser element is fabricated by the following method.
1) An epi-wafer is fabricated by stacking a plurality of layer on n-GaAs substrate 1. In this epi-wafer, n-GaAs (n=1×1018 cm−3) buffer layer 2 of thickness 0.5 μm, n-AlGaAs (n=1×1018 cm−3) lower clad layer 3 of thickness 1.5 μm, n-GaAs (n=3×1017 cm−3) lower optical confinement layer 4 of thickness 0.03 μm, p-In0.2Ga0.8As (p=3×1017 cm−3) active layer 5 of thickness 80 Å, p-GaAs (p=3×1017 cm−3) upper optical confinement layer 6 of thickness 0.03 μm, p-Al0.35Ga0.65As (p=1×1018 cm−3) upper clad layer 7 of thickness 1.2 μm, and p-GaAs (p=4×1019 cm−3) cap layer 9 of thickness 0.5 μm are successively stacked on the n-GaAs substrate 1.
2) A ridge mesa having a width of about 2 to 3 μm and length of about 800 μm is created on the epi-wafer using photolithography technique. As a result, length of the cavity of this semiconductor laser element becomes 800 μm.
3) The cap layer 9 from the anti-reflection side end facet, that is, from the laser-emission side end facet F1 is to a width of 25 μμm is removed by selective etching to obtain the current non-injection structure. The reference numeral F2 denotes the laser-reflection side end facet.
The cap layer may be removed using the semiconductor etchant disclosed, for example, in Japanese Patent Application Laid-Open No. 7-7004. This reference discloses an etchant that selectively etches only the GaAs layer when there exists layers of GaAs and AlGaAs. This method is therefore called selective etching. The etchant is prepared by adding a basic compound to a mixture of organic acid and hydrogen peroxide based mixture in such a manner that the pH of the mixture is between 6.0 and 8.0. The organic acid is, for example, citric acid.
Precisely, aqueous citric acid solution (1% by weight) and aqueous solution of hydrogen peroxide solution (30% by weight) are mixed at a volume ratio of 100:1. Ammonia is added to this mixture in such a manner that the pH of the mixture is between 6.0 and 8.0. Assume that the ratio of the etching rate of the p-GaAs cap layer 9 to that of the p-Al0.35Ga0.65As upper clad layer 7 is called as selection ratio. Then, the p-GaAs cap layer 9 is etched more effectively when the selection ratio is high and etching can be stopped exactly at the p-Al0.35Ga0.65As upper clad layer 7. Based on an experiment it was confirmed that the selection ratio is 85 when the pH of the etchant is 7.0. In an another experiment the p-GaAs cap layer 9 was removed using the etchant having the pH 7.0.
4) Both the surfaces of the ridge mesa were then covered with the SiN film 10. Finally, p-electrode 11 and n-electrode 12 are stacked to have the semiconductor laser element. This semiconductor laser element is also called a ridge waveguide type semiconductor laser element.
FIG. 11 shows a cross of another conventional semiconductor laser element. The difference between the semiconductor laser element shown in FIG. 11 and that shown in FIG. 9 is that the semiconductor laser element shown in FIG. 11 has p-Al0.15Ga0.85As resistance control layer 8 formed between the p-GaAs cap layer 9 and the p-Al0.35Ga0.65As upper clad layer 7. It is known that the resistivity of the p-GaAs cap layer 9 can be reduced by employing such a structure. FIG. 12 shows a cross section of the semiconductor laser element along the line A—A shown in FIG. 11. The current non-injection structure is formed by selectively etching a region from the laser-emission side end facet of the cap layer 9 and the resistance control layer 8. The width of this removed region is 25 μm. In other words, in this semiconductor laser element it is necessary to etch both the p-GaAs cap layer 9 and the p-Al0.15Ga0.85As resistance control layer 8 in one etching.
However, when both the p-GaAs cap layer 9 and the p-AlyGa1−yAs layer 8 are to be removed by one etching, depending on the composition ratio y of aluminum, a small portion of p-AlyGa1−yAs resistance control layer 8 is disadvantageously leftover as it is without being etched.
If even a small portion of the p-AlyGa1−yAs resistance control layer 8 remains then the end facet corrosion occurs when the laser output is increased. FIG. 11 and FIG. 12 show a portion 8a, of the layer 8, that remained above the upper clad layer 7 because of incomplete etching. When this portion 8a remains above the upper clad layer 7, which is a current non-injection layer, injection current flows into the laser-emission side end facet of the active layer 5 through this non-etched region 8a. When current flows in the active layer 5, the above-mentioned cycle of positive feedback occurs, and optical damage and end facet corrosion occur at the laser-emission side end facet.
Thus, with the conventional technology, it is possible to effectively etch only the GaAs layer when there are layers of GaAs and AlGaAs. However, it is almost impossible to completely etch the p-GaAs layer and the p-AlyGa1−yAs layer in one etching when there are layers of p-GaAs, p-AlyGa1−yAs, and p-AlxGa1−xAs.